The present invention relates to a radial bearing for journaling a rotating body and a transmission using the radial bearing, and more particularly to a radial bearing suitable for journaling a rotating body subjected to a fluctuating load and a transmission using the radial bearing.
Tin-based white metal has been commonly used as a sliding material of a radial bearing for bearing a rotating body. However, recently, engineering plastic having low friction, high heat resistance and high wear resistance properties has received attention as the sliding material, so that a high polymer composite resin material which is mainly made of thermoplastic polytetrafluoroethylene (tetrafluoroethylene resin), polyetheretherketone (PEEK) or polyimide, and added with glass fiber, carbon fiber, graphite, molybdenum disulfide or the like to improve the mechanical strength, sliding property, wear resistance property and the like becomes employed as a bearing sliding material especially for large rotating apparatuses.
Examples of a radial bearing using such resin material are described in JP-A-2000-249147 and JP-A-2000-81031. JP-A-2000-249147 discloses a radial bearing comprising a plurality of bearing pads and a metal filter interposed between a back metal, which is a reinforcing member constituting the bearing pads, and a resin sliding member. The back metal fixed with the metal filter is heated to a temperature equal to or larger than the melting point of the resin sliding member, and thereafter laminated with the resin sliding member thereon.
Further, in JP-A-2000-81031, a groove portion is provided on an inner surface of a sleeve bearing made of resin in order to reduce mechanical noise due to vibration of a shaft and the bearing and improve durability by a simple structure, without significantly reducing the clearance between the shaft and the bearing. By storing grease applied during the assembly in the groove portion, the grease can be supplied to the clearance portion for a long time to reduce noise and improve durability.
Although the radial bearing according to the above-described JP-A-2000-249147 specification has the advantage of improving adhesion between the resin sliding member and the back metal to prevent damage on a sliding surface and separation of the resin sliding member, the radial bearing structure is a tilting pad type bearing structure and thus, there is a need for further improvement with respect to load capability, while being superior in vibration property. That is, it is required that the radial bearing can stably support the rotating body even when the radial bearing is subjected to a large load exerted in a transmission, for example.
Further, the radial bearing according to JP-A-2000-81031 specification is a grease lubricated bearing for bearing a light load, unlike a oil lubricated bearing usable for a long period. Therefore, although the groove is formed, it is intended to store the grease while making little contribution to improvement of vibration property of the rotating body. Additionally, in the specification, it is not studied to support a large load by the radial bearing.
In view of the above-described prior arts, it is an object of the present invention to realize a radial bearing which can stably support a rotating body even under a light load and can prevent burnout of the bearing even under a high load, and a transmission using the radial bearing.
In order to achieve the above-described object, according to a first aspect of the invention, there is provided a radial bearing for supporting a rotating shaft, which has a substantial elliptical-cylinder-shaped inner circumferential surface and comprises at least two protruding portions extending in the axial direction on the inner circumferential surface of the bearing, wherein the protruding portions are symmetric with respect to the central axis of the bearing to stabilize the shafting.
In the bearing, it is preferable that the protruding portions are located in correspondence with a high specific load exerting area on the inner circumferential surface of the bearing to improve the load capacity. The protruding portions may be located in an area between the major axis of the ellipse, and 110 or 120 degrees therefrom in a certain circumferential direction on the inner circumferential surface. Additionally, it is preferable that the protruding portions are lubricating resin members provided on the inner circumferential surface of the bearing. The resin members may be made of one selected from the group of polyetheretherketone (PEEK), tetrafluoro-ethylene resin, and polyimide resin.
Further, it is preferable that the bearing can be divided into two parts substantially along the major axis of the ellipse. Still further, it is preferable that an oil groove is provided on an inner surface of the resin members in the vicinity of the major axis of the ellipse so as to extend in the axial direction of the bearing except for both axial ends of the bearing. Additionally, in the case of the two-part bearing, it may be built up so that the inner circumferential surface of the bearing has a symmetric shape with respect to the central axis of the rotating shaft.
In order to achieve the above-described object, according to a second aspect of the invention, there is provided a transmission comprising a gear shaft provided with a gear, a pinion shaft provided with a pinion engaging with the gear, and a plurality of bearings for supporting the gear shaft and the pinion shaft, wherein at least one of the plurality of bearings is the radial bearing according to the first aspect of the invention.